This invention relates to a power line communication systems and method of operation thereof and in particular to a system and method utilising Orthogonal Frequency Division Multiplexing (OFDM).
It is known to transport telecommunications signals over an electricity distribution or power transmission network. Delivering a telecommunications service in this manner is attractive as it avoids the need to install new cabling to each subscriber. By using existing electricity distribution cabling to carry telecommunications signals, significant cost savings are possible. International Patent Application WO94/09572 shows an electricity distribution network that carries telecommunications signals.
There are essentially two known methods for transmitting data over power lines. A first method uses the power signal itself, modifying the shape of the power signal at certain known points in response to the data that is to be carried. An example of this is shown in UK Patent GB 1,600,056. A second method uses a carrier signal having a different frequency from that of the power signal, the carrier signal being modulated by the data.
One of the problems with using power lines as a communications medium is that they are subject to noise and interference. A first type of noise is due to cables picking up radio signals such as broadcast AM radio signals and amateur radio band transmissions. Overhead cables are particularly prone to this type of noise. A second type of electrical noise is due to electrical equipment coupled to the power lines. Electric motors, thermostats and gas discharge lighting are particularly prone to generating noise. Noise propagates along the power lines and combines with communications signals. The level of noise can be high enough, and persist for long enough, to corrupt communications signals.
Noise can impose constraints when using the power line to carry data. The aforementioned patent application WO94/09572 shows an electricity distribution network which carries telecommunications signals and teaches that the amount of noise entering the network from the premises can be reduced by installing conditioning elements at each premises. This requires a considerable amount of work and expense.
Power line transmission is one of several ways for delivering communications to subscriber premises and is therefore sensitive to competition with existing copper wires and more recent alternatives such as optical fibre/coaxial cable delivery and fixed radio access techniques. Therefore, there is a desire to provide a system that is capable of delivering an acceptable quality of service at a cost that is attractive to a subscriber. One of the aspects of a power line transmission system that is particularly sensitive to cost is the customer premises equipment (CPE). The customer premises equipment must be capable of delivering an acceptable quality of service and interfacing to equipment in a subscriber""s premises.
A number of different modulation schemes may be used in the provision of data services over power line. The applicant has found a number of schemes to be inappropriate:
Spread spectrum in the context of access by time division is not well matched to this application because of its low spectral efficiency.
High order Quadrature Amplitude Modulation QAM (16 QAM and above) is spectrally efficient but relatively insensitive i.e. it requires a quite high signal to noise ratio for reliable reception.
Binary Phase-Shift Keying BPSK either employing raised cosine filtering or minimum shift keying is not sufficiently spectrally efficient for a robust solution to this application.
However, the applicant believes two schemes to be acceptable, as follows:
Quadrature Phase-Shift Keying QPSK on a single carrier with raised cosine filtering has sufficient spectral efficiency for including significant coding overhead to strengthen the link""s robustness. It does however require adaptive equalisation (albeit with very slow adaptation speed requirement) to cope with frequency selective fading.
Orthogonal Frequency Division Multiplexing OFDM with QPSK modulation on the sub-carriers results in a spectral efficiency which is comparable to the above with contiguous frequency allocations. However, this technique is potentially superior in situations where the allocated spectrum is non-uniform and non-contiguous. There is no need for tapped delay line adaptive equalisers; instead, a simple channel compensation algorithm has to be included. Coding is required to make the radio interface robust.
The use of OFDM provides a superior flexibility to fit into non-uniform and non-contiguous frequency allocations, while maintaining reasonable spectral efficiency. This results form the intrinsic nature of OFDM which is composed of a large number of simultaneously transmitted sub-carriers which are staggered in frequency each individually occupying a low bandwidth, as illustrated in FIG. 1.
The scheme""s flexibility comes about from the ability to designate which sub-carriers are to be activated and which are not. Regarding spectral efficiency, the signal composition results in an intrinsic spectrum fall-off outside of the active bandwidth commensurate with the bandwidth of each sub-carrier rather than with the total spectrum width. Thus relatively low excess bandwidths can be achieved.
Therefore, the spectral attributes of OFDM represent the major advantage in favour of its selection for use in power line telecommunication systems. However, despite the clear advantages that could be achieved by the use of ODFM or CODFM in power line telecommunications systems there are presently no commercially available power line telecommunications systems which utilise an OFDM or COFDM modulation scheme.
It is submitted that one reason for the non-use commercially at present of OFDM or COFDM is the problem of impulse noise, as mentioned above. The applicant has discovered that large narrow regular spikes of impulsive interference can arise, for example, from fluorescent lights, which can conceivably flood all the sub-carrier demodulators with sufficient noise to prevent reception of complete symbols, as illustrated in FIG. 2. This noise is aligned with mains frequency.
The present invention seeks to minimise or overcome the above problem, and other problems that will become apparent from the following description.
According to a first aspect of the present invention there is provided an OFDM power line modem receiver comprising a clipping system adapted to clip an incoming OFDM data waveform, which includes a regular impulsive noise component, so as to reduce the level of said noise on the waveform.
Preferably, when the receiver is arranged to receive a coded OFDM signal (COFDM), wherein the clipping level can be altered, enabling the clipping level to be set such that the COFDM signal is not corrupted beyond the error correcting capabilities of the coding scheme utilised while reducing the power in the noise signal thus preventing corruption of the data in the waveform upon detection.
Preferably, the receiver further comprising a Fourier Transform element and a gain control element, wherein the clipping system and the gain control element act upon a received signal prior to its application to the Fourier Transform element.
This prevents the majority of the impulse power spreading across the OFDM frequency cells.
According to a second aspect of the present invention there is provided method of operating an OFDM power line modem receiver comprising the step of clipping an incoming OFDM data waveform, which includes a regular impulsive noise component, so as to reduce the level of said noise in the waveform.
Preferably, the signals are carried over the power line on radio frequency (RF) carriers.
Most preferably, the signals are carried over the power line in the frequency band greater than 1 MHz. Preferably, the signals comprise data packets which are transmitted with a bit rate of at least 500 kbps.
According to a third aspect of the present invention there is provided an OFDM power line communications system comprising:
a power line for distributing electricity to a plurality of premises; and,
a communication station coupled to the power line at one of the premises, which station uses a part of the power line external to the premises as a communications medium,
the communications station including a receiver comprising a clipping system adapted to clip an incoming OFDM data waveform, which includes a regular impulsive noise component, so as to reduce the level of said noise on the waveform.
Preferably, the communications system is operable to select a bit rate for transmitting/receiving communications signals over the power line from a plurality of different bit rates.
Preferably, the communications system further comprising a management unit, which is responsive to management signals received over the power line.
Operating at this higher frequency band allows data communications to be transmitted at a higher bit rate because of the greater available bandwidth, in the range 2-10 MHz. It has been found to be particularly advantageous to transmit communications signals within the frequency band 2-6 MHz and in particular within the bands 2.2-3.5 MHz and 3.8-5.8 MHz. These bands fall between the medium wave and short wave bands used for broadcast radio transmissions and avoid the radio amateur band at 3.5-3.8 MHz. However, the present invention is not restricted to use within these bandwidths.
The applicant has found that with presently available OFDM systems packet length of around 5 ms, and preferably 2-5 ms offers advantageous results as the packet is long enough to carry a useful data payload and therefore provide a high system throughput, but short enough to statistically have a high chance of fitting between high-level noise bursts. However, it is an advantage of the present invention that longer packet lengths can be used, because of the reduction in noise levels. Thus a larger number of narrower bandwidth channels can be utilised.
Preferred features may be combined as appropriate, and may be combined with any of the aspects of the invention, as would be apparent to a person skilled in the art.
The data that is carried in this system can be used for computer applications such as home working, file transfer, internet access and a wide range of other applications.